The present invention relates to an impact micro-positioning actuator providing for precision micro-positioning at temperatures ranging from ambient to cryogenic.
Micro-positioning actuators have many applications, and are used generally for optical or mechanical systems requiring precise alignment. A most demanding application is in outer space on large devices of lightweight construction. Such devices must rely on active control to maintain required dimensional stability. Micro-positioning actuators have been used, for example, to produce large mirrors suitable for use in an orbiting telescope, wherein a thin, flexible mirror element is rigidly attached at many points to a stiff carbon composite structure through corresponding micro-positioning actuators. Such systems must be operable at the cryogenic temperatures of space, and it is desirable to be able to operate the devices at ambient temperature on earth as well, to facilitate testing the systems. Micro-positioning actuators used for one-time or repeated adjustments of cryostats must also be operable at cryogenic temperatures.
It is always important to provide micro-positioning actuators with sufficient durability, reliability, and precision for the application, and this is especially difficult where the actuators are exposed to extremes of temperature. It is also important to minimize hysteresis in micro-positioning actuators that must be cycled, and to minimize their cost and weight, the latter being especially important in space applications. Another highly desirable feature in a micro-positioning actuator employed in outer space is the ability to hold a position without the application of power, to conserve power as well as to prevent the production of excess heat which is more difficult to dissipate in outer space due to the lack of a convective heat loss mechanism.
A commonly used precision actuator is the stepper motor. Stepper motors, however, have significant disadvantages as micro-positioners, especially for applications such as described above where high precision and low weight are of premium importance. In particular, very small stepper motors provide a limited number of steps per revolution, such as about 16, so that a gear train is also required for micro-positioning. The gear train adds cost and weight, and introduces random positioning errors, particularly hysteresis due to backlash. Moreover, a stepper motor adapted for use at both ambient and cryogenic temperatures, though possible, would be relatively expensive.
Another strategy for micro-positioning is the piezoelectric actuator such as described in Luecke, et al., U.S. Pat. No. 5,410,206 (“Luecke”). A piezoelectric element is mounted in a frame having a pair of jaws between which is disposed a threaded output shaft to be driven by the jaws. The piezoelectric element is operative to effect reciprocating motion of at least one of the jaws. The reciprocating motion of the jaw against the output shaft is converted to an incremental rotary motion by moving the jaw relatively slowly in a first direction such that the coefficient of friction between the shaft and the jaw overcomes the inertia of the shaft, and moving the jaw relatively fast in a second direction such that the inertia of the shaft prevents it from following the jaw, so that the shaft slips between the jaws to preserve the preceding incremental motion. Accordingly, a cyclic electrical signal applied to the piezoelectric element that is slowly rising and rapidly falling causes the shaft to rotate in one direction, and where the signal is rapidly rising but slowly falling, the shaft rotates in the opposite direction.
Although the Luecke device is mechanically simple and therefore can be implemented at lower cost than stepper motors, it has serious drawbacks for use at cryogenic temperatures. Particularly, the piezoelectric effect at cryogenic temperatures is an order of magnitude smaller than the effect at room temperature. Therefore, a complex calibration and compensation scheme would be required to produce a system that operates repeatably at both temperature extremes. Moreover, piezo-materials that function well at room temperature do not function well at cryogenic temperatures and vice versa. For these reasons, it is difficult or impossible to scale a piezoelectric micro-positioned to repeatably position a range of loads over a range of temperatures.
The present inventors have proposed an alternative impact micro-positioning concept in a paper entitled “A Linear Micro-Positioning Actuator for Ambient and Cryogenic Operation.” The actuator was to take the form of a shaft threaded into a nut which is impacted with a small mass to cause momentary rotation. A torsional spring was proposed to restore the nut to its initial position after each impact. The shaft was preloaded to obtain a particular frictional torque between the nut and the shaft. The preload was proposed to set a frictional force between the shaft and nut that would be lower than the force required to accelerate the shaft at the high initial angular acceleration of the nut on impact. As a result, the angular position of the shaft would lag behind the nut after impact. However, as the nut is thereafter decelerated by the torsion spring, a point is reached where the force required to turn the shaft at the same rate falls to a level at or below the frictional force, so the shaft and nut move together as the nut returns to its original position. The result of the cycle is that some of the initial advancement of the nut relative to the shaft is preserved.
It was further proposed that the speed and momentum of the impacting mass could be adjusted to vary the relative motion of the shaft and nut and hence the step size. Motion in two directions was proposed to be provided by two separate impactors. A prototype was fabricated and was reported to provide repeatable and reliable steps of from 10 to 100 nm at the 77 degrees Kelvin.
The impact micro-positioning device solved the problems inherent in the Luecke piezoelectric actuating device. However, subsequent testing revealed that the device did not perform reliably over the required ranges of temperature, stroke and direction required for many applications, including applications in outer space. Accordingly, there is a need to refine the concept of an impact micro-positioning device in a number of its aspects to realize the full benefit of the concept.